The present invention relates to the hydrotreatment of tars derived from the pyrolysis of coals to saturate and reduce the molecular weight of the tar constituents while removing bound sulfur, nitrogen and oxygen. Coal tars formed from pyrolysis and other thermal decomposition techniques can be upgraded by treatment with hydrogen at elevated temperatures and pressures over sulfided nickel, molybdenum and cobalt containing catalysts.
Direct contact of the substantially liquefied tars with the catalyst in the hydrogenation zone suffers from the difficulty that at the higher temperatures, the residence times necessary to achieve hydrogenation of the unsaturates in the coal tars favors the tendency of coke formation from the heavier molecules in the feed stock. Coke, in this instance, is a high molecular weight crosslinked material formed by the cross-linking of molecules through sites of unsaturation to yield a polymer which will further condense to free carbon before melting occurs. The coke which forms settles on and rapidly occludes the catalyst.
The coke forming tendency can be reduced by increasing effective hydrogen pressure. This, however, leads to increased hydrogen consumption in undesired side reactions leading to total saturation of the molecules. This is undesired, as the most utile product of the hydrogenation are low boiling hydrocarbons boiling at a temperature from about 300 to about 750.degree. F and which are highly aromatic in nature. The increased hydrogen consumption is wasteful of a costly manufactured commodity.
A need exists, therefore, for a hydrogenation process which minimizes coke formation without sacrificing the quality of the hydrocarbons as the desired product of hydrogenation.